The present invention relates to an exhaust gas recirculation (EGR) system for controlling the flow of exhaust gas from an exhaust gas manifold to an intake manifold of an internal combustion engine, and more particularly, to an improved method for controlling such an EGR system.
Although the use of the present invention is not limited to any particular type or configuration of engine, its use is especially advantageous in connection with a heavy duty diesel engine, and the invention will be described in connection therewith. Furthermore, although the present invention may be utilized advantageously in connection with the control of various engine elements, such as electromagnetically-operated engine poppet valves on camless engines, and control rods for VGT (variable geometry turbine) systems, the invention is especially advantageous when utilized in connection with an EGR system, and will be described in connection therewith.
EGR systems are utilized in automotive vehicles (i.e., including both passenger cars and trucks) in order to help reduce engine emissions, and are desirable especially on heavy duty diesel engines. Such EGR systems typically utilize an EGR poppet valve that is disposed between the engine exhaust manifold and the engine intake manifold. The EGR poppet valve is operable, when in an open position, to permit recirculation of exhaust gas from the exhaust manifold back into the intake manifold. As is well know to those skilled in the art, such recirculation of exhaust gasses is helpful in reducing various engine emissions. As is also well known to those skilled in the art, when the engine is operating under relatively heavy torque loads (such as while accelerating or shifting gears at low speeds), the EGR valve will typically be closed, or nearly closed, whereas, when the engine is operating under relatively lighter torque loads (such as at steady state engine speed, in a higher gear ratio), the EGR valve will typically be fully open, or almost fully open.
An electromagnetic actuator is preferably employed for moving the EGR poppet valve between its open and closed positions, because the recirculation of exhaust gasses is appropriate and helpful only at certain times during the operation of the engine, in accordance with the previous discussion, and it is desirable to be able to change the position of the EGR poppet valve very quickly to adjust to varying vehicle and engine operating conditions. EGR valves of the type with which the present invention may be utilized are illustrated and described in U.S. Pat. Nos. 5,937,835 and 6,102,016, both of which are assigned to the assignee of the present invention and are incorporated herein by reference.
Electrically actuated EGR valve systems preferably utilize software-implemented control logic, such that the EGR poppet valve is operating under closed loop control when the EGR poppet valve is being moved from a closed position to an open position, and when it is being moved from an open position to a closed position. As used herein, the term xe2x80x9cclosed loopxe2x80x9d in regard to the control of the EGR poppet valve will be understood to mean that the control logic is constantly xe2x80x9creadingxe2x80x9d the position of the valve, and utilizing the resulting position signal as part of the feedback to the control logic. The closed loop control logic controls electric current to an electric motor which serves as the actuator to move the EGR poppet valve, and control the opening/closing position thereof. In such systems, the control logic typically generates pulse width modulated (PWM) signals to power the actuator motor, and modulate the movement of the EGR poppet valve, moving it from one position to another.
As is also well know to those skilled in the art of position control using DC motors, it is not sufficient, when designing the control logic for an engine component such as an EGR poppet valve, to merely establish a baseline relationship of EGR poppet valve position as a function of control current, and thereafter assume that the position-versus-current relationship will remain constant (i.e., equal to the baseline relationship). For example, it is now well know to those skilled in the art of controlling electrically actuated devices to adjust the gain compensation within the control circuit as a function of the ambient temperature of the device being controlled. In the course of developing the commercial embodiment of an EGR poppet valve system of the type to which the present invention relates, the assignee of the present invention has taken into account the typical, well known system variables (e.g., fluctuations in system voltage, ambient temperature, etc.), and has built into the EGR system control logic the appropriate compensation for variations in such factors. However, it has been observed by the assignee of the present invention that there have still been aspects of the overall EGR system performance, on the developmental systems, which have not been fully acceptable.
As a result of the development of the present invention, it has been observed by the inventor of the present invention that the performance of the EGR system can change substantially, over a relatively short period of time, especially when the EGR system is operating under conditions such that the EGR poppet valve remains open during a major portion of a given time period. It has now been determined that at least one likely cause of such changes in the performance of the EGR system relates to the system xe2x80x9cfrictionxe2x80x9d, and especially, the static friction (i.e., the friction when the system is not moving) which must be overcome to achieve initial movement of the poppet valve. The friction being referred to hereinabove would include that in the gear train or drive train between the electromagnetic actuator (motor) and the poppet valve, as well as that associated with the engagement of the poppet valve stem and the bore in which the stem reciprocates. In some EGR systems, there may also be seals, or other elements which provide a frictional xe2x80x9cdragxe2x80x9d which resists movement of the poppet valve.
Unfortunately, it has now been determined that, not only does the system have to overcome the static friction in order to begin to move the EGR poppet valve, but also, the total amount of the static friction which must be overcome can change substantially. It is now believed that a major cause of the changing static friction is the exhaust gas soot, and the various other contaminants from the EGR gas (all of which are hereinafter, for simplicity, collectively referred to as xe2x80x9csootxe2x80x9d), which build up at various locations, such as on the valve stem. If the EGR poppet valve remains open for an extended period of time, such as an hour, there may be enough of a build-up of soot to change the static friction of the system by 20 or 30 percent, or more, thus requiring substantially more electric power than usual to overcome the friction and achieve initial movement of the EGR poppet valve.
However, as a further complication in attempting to compensate for the build-up of soot, and the resulting increase in the static coefficient of friction (xe2x80x9cCOFxe2x80x9d), it is also known that during operation of the vehicle engine, the built-up soot can get burned-off, thus decreasing the static COF. In other words, the static COF goes up and down, as a function of the driving cycle. Furthermore, when the static COF is relatively high (because of soot as explained previously) and the difference between the static COF and the dynamic COF (i.e., when the system is moving) becomes fairly large, controlling accurately the movement of the EGR valve becomes even more difficult, as there is a tendency for the valve to xe2x80x9covershootxe2x80x9d its commanded position. This is true because a relatively higher current is needed to overcome the static friction, and get the valve moving, but then the current to the motor is excessive, once the valve begins to move, in view of the much lower dynamic COF . The overshoot problem typically means that it takes a longer time to get the EGR valve to the desired position, which may result in more exhaust gas being released than was intended. Also, in the event of overshoot of the EGR valve position, there can be unintentional engagements with mechanical stops which comprise part of the system, causing excessive wear and reducing the durability of the EGR assembly.
Accordingly, it is an object of the present invention to provide an improved control member system, and an improved method for controlling such a system, which achieves a greater consistency and predictability in the operating performance of the system.
It is a more specific object of the present invention to provide such an improved method of controlling an EGR valve system which substantially eliminates one of the major sources of variation in overall system performance.
It is another object of the present invention to provide an improved method of controlling such a system, which accomplishes the above-stated objects by compensating for variations in system friction over a period of time.
The above and other objects of the invention are accomplished by the provision of an improved method of controlling the movement of an assembly in an internal combustion engine. The assembly includes a control member moveable between a closed position, blocking communication from a first engine gas passage to a second engine gas passage, and an open position. The assembly further includes housing means, the control member being disposed within the housing means for reciprocable movement therein. An electromagnetic actuator operably associated with the housing means has an actuator output. A drive train is operable to transmit movement of the actuator output into reciprocating movement of the control member in response to changes in an electrical input signal, the method of controlling the movement comprising the steps of generating a compensator gain value to modify the electrical input signal.
The improved method of controlling the movement is characterized by providing a position sensor operable to sense a position of the control member and generate a position signal representing instantaneous control member position. The next step is storing a first relationship of the electrical input signal required to change the instantaneous control member position. During ongoing operation of the internal combustion engine, the next step is generating a then-current, second relationship of the electrical input signal required to change the instantaneous control member position. Next, the method compares the second relationship to the first relationship and generates a corresponding difference factor, and uses that factor to modify the compensator gain value correspondingly.